1. Field of the Invention
The present invention relates to a high throughput holographic lithography system for generating interferometric patterns suitable for selectively exposing a photosensitive material and, more particularly, to an easily reconfigurable lithographic patterning tool employing a fiber optic beam delivery system and a prism optically coupled to the photosensitive material.
2. Description of the Related Art
Holographic or interferometric lithography has been proven in laboratory environments to be feasible for generating patterns of light suitable for exposing photosensitive materials in the manufacture of devices having sub-micron features. Holographic lithography exploits the mutual coherence of multiple optical beams derived from a single light source such as a laser. The laser beams are made to overlap in some region of space and interfere to produce patterns of light and dark areas that repeat on a scale proportional to the wavelength of the light source. The interferometric patterns of light are recorded in a photosensitive medium, such as photoresist, properly positioned within the region of space. Conventional contact or projection photo masks are not required; thus, holographic lithography is known as a "maskless" lithography technique. Additionally, by exploiting inherent photoresist and etching process non-linearities, a variety of surface relief structures can be generated with no change in the optical configuration.
There are a number of applications that would benefit from the sub-micron sized structures that can be produced using interferometric patterns generated from holographic lithography techniques. For example, holographic lithography can be used to produce improved distributed feedback (DFB) gratings which are employed in the telecommunications field. More specifically, the telecommunications market is experiencing rapid growth and prosperity due to advances in fiber-optic technology and the advent of wavelength division multiplexing (WDM). WDM techniques allow the efficient combination of multi-channel (multi-carrier frequency), high-bit rate signals onto a single optical fiber. Solid state laser sources operating about wavelength bands centered at 1310 nanometers (nm) and 1550 nm are employed to transmit digital information at rates as high as 2.5 gigabits per second. These solid state lasers emit light in a multi-longitudinal mode wherein numerous narrow band wavelengths are clustered about the center wavelength. Interference between these wavelengths limits the number of channels that can be transmitted along an optical fiber.
Distributed feedback solid state lasers incorporate DFB gratings within the lasing medium to act as filters to limit the lasing output to a single narrow-banded mode. This narrow-band operation is required for long haul and high-speed telecommunications. Such gratings are typically produced via a phase mask technique or by e-beam lithography. Both techniques suffer from practical limitations such as small field size, stitching errors, short mask lifetimes, low throughput, and high cost. Thus, a practical, low cost, reliable, easily reconfigured production tool is needed to more efficiently produce DFB gratings.
Holographic, or interferometric lithography has been used experimentally to produce DFB gratings with the advantages of large area, high throughput, no stitching errors and no photomasks. The technology enables the realization of a non-contact, non-scanned, maskless pattern generator. No intermediate photomasks are required. The principal advantages of holographic lithography include: sub-half-micron resolution; nearly unlimited field size; a lensless configuration; the capability to form patterns on arbitrary surfaces; a cost effective mechanism for obtaining high yield and throughput; and compatibility with all current semiconductor, photoresist and mask production technologies.
The line arrays, or gratings required for distributed feedback are derived by recording a classical two-beam interference pattern. FIG. 1 depicts a typical laboratory system 10 for exposing a photosensitive material with a two-beam interference pattern. A laser source 12, such as a single frequency argon-ion laser, produces a laser beam that is split into two substantially equal beams by a beam splitter 14. The two beams are incident on respective turning mirrors 16 and 18 which direct the two beams toward respective spatial filters 20 and 22. Each of spatial filters 20 and 22 causes its respective laser beam to diverge, such that two divergent illuminating beams 24 and 26 are respectively projected from the two spatial filters. The points from which the illuminating beams are emitted from the spatial filters lie in a point source plane 28, and the spatial filters are oriented such that the illuminating beams overlap at some distance from the point source plane to produce a patternable volume 30. A wafer or panel 32 coated with a photosensitive material such as a photoresist is placed in the patternable volume 30 with the surface of the photosensitive material lying in a recording plane 34 that is substantially parallel to the point source plane 28. Typically, the distance between the recording plane and the point source plane is on the order of at least one meter. Exposure timing and duration can be controlled with an electronic shutter/timer 36 lying in the optical path of the laser beam (e.g., between the laser source 12 and beam splitter 14).
As shown in FIG. 2a, wafer 32 includes at least a substrate 40 coated with a photoresist layer 42. Photoresist layer 42 is subjected to two-beam holographic exposure by illuminating beams 24 and 26. Specifically, beams 24 and 26 create an interference pattern in the recording plane 34 in the form of a grating of parallel lines of high light intensity alternating with parallel lines of low light intensity. The light intensity varies sinusoidally in the recording plane 43 in the direction perpendicular to the orientation of the lines of the grating pattern.
The two expanded beams 24 and 26 overlap in the recording plane 34 to form the grating pattern and selectively expose the photoresist layer 42. This exposure creates a latent image in the photoresist consisting of parallel lines in an array. The sinusoidal intensity distribution of the interference pattern has a modulation contrast of four to one and a peak-to-peak distance, or pitch, given by the relation: EQU .LAMBDA.=.lambda..sub.o (2n.sub.i sin .theta..sub.i) (1)
where .lambda..sub.o is the vacuum wavelength, .theta..sub.i is the incident beam angle measured from the surface normal, and n.sub.i is the index of refraction of the incident medium.
After a suitable non-linear photoresist development process, a grating in the photoresist is produced which can then be employed as a mask during a subsequent etch process which replicates the grating in the distributed feedback material. Specifically, as shown in FIG. 2b, the developed photoresist layer 42' is a grating corresponding to the illumination grating pattern, with parallel lines of developed photoresist separated by linear regions where the photoresist has been removed. An etching process, such as reactive ion etching (RIE), can then employ the photoresist grating as a mask to reproduce the grating pattern in the substrate 40 (see FIG. 2c).
A graphical representation of the intensity distribution and the resulting calculated photoresist profile predicted from the measured response characteristics of a commercially available resist are respectively shown in FIGS. 3a and 3b. The nature of the photoresist grating mask that is recorded using two-beam interferometric lithography can be clearly seen in the scanning electron micrographs (SEM) of FIGS. 4a(0.8 .mu.m pitch, 1.2 .mu.m depth) and 4b (resist on Si, 460 nm pitch, .about.200 nm CD) showing edge profile cross-sections of grating masks. Lines in the photoresist slightly less than 0.4 micron in width are located on 0.8 and 0.75 micron spacings. This grating pitch can be continuously varied simply by changing the angle between the two interfering beams. Line width uniformity over the patterned area (e.g., 60 mm) is directly impacted by the exposing beam intensity variations. These intensity variations are minimized through careful implementation of the well established holographic technique of exposure with highly divergent optical beams derived from spatial filters. Additionally, macroscopic photoresist thickness variations impacting line width uniformity can be found due to multiple reflected beam interference in the recording plane. This more subtle problem can be eliminated either by choosing the thickness of the initial photoresist layer to act as a single layer anti-reflection coating or, with highly reflecting surfaces, by employing an additional anti-reflection coating.
Holographic or interferometric lithography has application in other technologies as well. For example, holographic lithography has been proven in laboratory environments to be feasible for generating the large-area, sub-micron sized periodic structures suitable for manufacturing flat panel displays based on distributed cathode field emission display (FED) technology, a strong competitor in the flat panel display market currently dominated by liquid crystal display (LCD) manufacturers. A FED is a distributed cathode, flat panel analog to the well known cathode ray tube (CRT). Essentially, billions of miniature electron "gun" cathodes are distributed spatially over the surface of a display substrate. The most critical step in the fabrication of the FED distributed cathode matrix is patterning of an array of holes or wells in which emitter cones are grown. Conventional photo-lithographic techniques such as shadow masking (contact printing), optical projection, and electron or laser beam direct writing have proven ineffective in producing commercially useful hole arrays. As described in U.S. patent application Ser. No. 09/202,367, the disclosure of which is incorporated herein by reference in its entirety, holographic lithography holds promise to realize the potential of FED technology by providing an inexpensive, high speed, production environment tool, incorporating a patterning technology capable of producing large-area, high-density, sub-micron diameter hole arrays with few defects and at low cost.
Other useful surface relief structures can be patterned using holographic lithography, such as a "motheye" or sub-wavelength-structure (SWS) surfaces. Motheye surface structures have been shown to be effective for nearly eliminating the reflectance of light from an optical interface such as between air and a window or a refractive optical element. The term "motheye" is derived from the insect's eye, a natural analog; the eye of a nocturnal insect (e.g., a moth) reflects little or no light regardless of the light wavelength or the angle at which incident light strikes the eye surface. The eye surface functions in a manner similar to a graded index material, essentially allowing the smooth transition between media with differing bulk density. To avoid diffraction effects, synthetic motheye surfaces must be fabricated with feature sizes and spacings smaller than the wavelength of light incident upon the surface. For most infrared or visible wavelength applications, this necessitates structure spacings in the sub-micron to sub-half micron range, patterned over the entire surface (e.g., window or optic area). A means for manufacturing motheye structures in high volumes and over large areas is not available in the prior art and a variety of products could benefit from the increased ruggedness and anti-reflective performance afforded by motheye surfacing over large areas.
Holographic lithography also has application in the manufacturing of liquid crystal displays (LCDs). Liquid crystals (LCs) are anisotropic molecules which can affect the properties of light with which they interact, and, under the influence of an electric field, can vary the magnitude of this effect. LCDs are formed by the creation of a cell, typically constructed using glass, within which the LC molecules are confined. The term "crystal" refers to the structure or ordering of the LC molecules into a definable or measurable state typically found with molecules in a solid state. This artificially created ordering is accomplished by depositing thin layers of material on the boundaries of the cell, which either physically, or electrostatically force the LC molecules to preferentially align in one direction. The "alignment layers" as they are known in the art, are typically processed using a physical rubbing or buffing technique comprising a spinning drum or cylinder and rolling it over the cell substrate coated with alignment material. High levels of hazardous static charge and spreading particulate (from the rubbing material) are generated during this process; in addition, manufacturing yields can be improved.
Holographic lithography can potentially increase LCD manufacturing yield by providing a non-mechanical, non-contact alignment layer formation process wherein alignment material layers are produced by patterning surface structures. This technique avoids problems with static charge and is compatible with existing manufacturing equipment and environments. Specifically, holographic lithography techniques can be used to produce surface structure LC alignment layers with the enhanced anti-reflective properties of motheye surfaces. By producing sufficient asymmetry in the surface structures, the patterning process allows control of both angular rotation and angular tilt of the LC molecules with respect to the cell walls.
Holographical lithography demonstrations in the laboratory have employed UV wavelength light derived from an argon ion gas laser which is highly automated and reliable, making it a good choice for a manufacturing tool. A wavelength in the deep blue spectral range is also available with argon ion gas lasers. UV wavelength light has a number of advantages. A large variety of photoresists possess high sensitivity to energy in the near-UV spectral range, whereas the number of photoresists sensitive to energy having a visible blue wavelength is more limited, and, in those photoresists, sensitivity is typically lower. Further, the shorter UV wavelength permits formation of smaller features relative to a comparable system employing blue wavelength light. For example, in the case of two-beam exposure for forming DFB gratings, it can be seen from equation (1) that the a longer wavelength produces a proportionally greater grating pitch.
However, for a number of reasons, the blue wavelength would be a more practical choice for a manufacturing tool. First, the divergence of an optical beam for a given waist or aperture size decreases for shorter wavelengths, a relationship having a direct impact on field size in the proposed system. For a given aperture size, the field size is 30% larger for 458 nm (blue) light than for 351 nm (UV) light. Second, alignment and maintenance of a holographic setup is greatly simplified when operating with a visible, relatively eye-safe light source. Third, laser lifetime for the argon-ion gas laser is dramatically reduced when operated at near-UV wavelengths. Consequently, a production environment laser is expected to last up to two times longer when operated at 458 nm (blue light). Further, in order to have a flexible and easily reconfigurable beam delivery system (e.g., adjustable beam positions and angles), it would be desirable to employ optical fibers to deliver plural coherent beams from a common source to the point source plane (as opposed to beams delivered via free space and discrete components such as mirrors and filters). As explained in greater detail below, in addition to being easily repositioned, optical fibers can act as natural spatial filters, causing the laser beams to diverge when emitted from one end, thereby eliminating the need for separate spatial filters. However, to date, the specialized optical fibers suitable for use in such a beam delivery system exhibit inferior, unstable, and impractical performance when guiding UV wavelength light. Thus, for this additional reason, blue wavelength light is desirable.
However, as previously explained, the laser light in the blue region of the visible spectrum has a longer wavelength than UV light. All other parameters being equal, the larger wavelength of blue light results in a corresponding enlargement of the interference pattern relative to that produced by UV light. This enlargement is particularly problematic in the context of producing DFB gratings. To be useful, DFB gratings must be produced with a grating pitch as small as approximately 200 nm. Referring to the equation (1), it can be seen that the range of values for the grating pitch is limited by the physical characteristics of the optical setup. The smallest pitch attainable is given by .lambda..sub.2 /n.sub.i when the incident beam angles are 90.degree. with respect to normal. Whereas 200 nm pitch sizes can be achieved with 351 nm UV wavelength light, 457.9 nm blue wavelength light cannot produce 200 nm gratings in a comparable system.
Thus, while the desire to have an easily adjustable optical-fiber-based system and other considerations lead to the selection of a visible (blue) wavelength, blue wavelength light results in a somewhat larger interference pattern that is unacceptable in certain applications. Accordingly, there remains a need in the art to address the limitations imposed by employing blue light.